Vortex Induced Vibration Optimizing System

ABSTRACT

There is disclosed a system comprising a structure, a vortex induced vibration monitoring system, adapted to monitor a vortex induced vibration level of the structure, a tensioner connected to the structure, and a controller adapted to calculate a tension on the structure to optimize the vortex induced vibration value of the structure.

FIELD OF INVENTION

The present disclosure relates to systems and methods for optimizing thevortex induced vibration of a substantially cylindrical structure in abody of water.

BACKGROUND

U.S. Pat. No. 6,695,540 discloses a vortex induced vibration suppressorand method. The apparatus includes a body that is a flexible member of apolymeric (e.g. polyurethane) construction. A plurality of helical vaneson the body extend longitudinally along and helically about the body. Alongitudinal slot enables the body to be spread apart for placing thebody upon a riser, pipe or pipeline. Adhesive and/or bolted connectionsoptionally enable the body to be secured to the pipe, pipeline or riser.U.S. Pat. No. 6,695,540 is herein incorporated by reference in itsentirety.

U.S. Pat. No. 6,561,734 discloses a partial helical strake system andmethod for suppressing vortex-induced-vibration of a substantiallycylindrical marine element, the strake system having a base connected tothe cylindrical marine element and an array of helical strakesprojecting from the base for about half or less of the circumference ofthe cylindrical marine element. U.S. Pat. No. 6,561,734 is hereinincorporated by reference in its entirety.

U.S. Pat. No. 6,223,672 discloses an ultrashort fairing for suppressingvortex-induced vibration in substantially cylindrical marine elements.The ultrashort falling has a leading edge substantially defined by thecircular profile of the marine element for a distance following at leastabout 270 degrees thereabout and a pair of shaped sides departing fromthe circular profile of the marine riser and converging at a trailingedge. The ultrashort fairing has dimensions of thickness and chordlength such that the chord to thickness ratio is between about 1.20 and1.10. U.S. Pat. No. 6,223,672 is herein incorporated by reference in itsentirety.

Referring to FIG. 1, there is illustrated system 100. X axis 102, Y axis104, and Z axis 106 are all defined. System 100 includes vessel 110floating in water 112. Cylindrical structure 114 is connected to vessel110, and cylindrical structure 114 goes to bottom 116 of water 112.Current 118 a, 118 b, and 118 c are all traveling in the X direction,and encounter cylindrical structure 114. Vortexes 120 a, 120 b, and 120c are caused by the interaction of currents 118 a-118 c with cylindricalstructure 114. Vortex induced vibrations (VIV) 122 a, 122 b, and 122 care caused by interaction of currents 118 a-118 c with cylindricalstructure 114.

There is a need in the art for systems and/or methods to optimize VIV ofstructures exposed to a current or wind.

SUMMARY OF THE INVENTION

One aspect of the invention includes a system comprising a structure, avortex induced vibration monitoring system, adapted to monitor a vortexinduced vibration level of the structure, a tensioner connected to thestructure, and a controller adapted to calculate a tension on thestructure to optimize the vortex induced vibration value of thestructure.

Another aspect of the invention includes a method of controlling vortexinduced vibration of a structure in a body of water comprisingmonitoring a level of vortex induced vibration in the structure, andadjusting the tension in the structure to minimize the level of vortexinduced vibration.

Another aspect of the invention includes an apparatus for minimizingvortex induced vibration in a structure comprising a means forcalculating the level of vortex induced vibration of the structure, ameans for calculating an optimum level of tension in the structure tominimize the vortex induced vibration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a vessel floating in water connected to a cylindricalstructure.

FIG. 2 illustrates a vessel floating in water connected to a cylindricalstructure.

FIG. 3 illustrates a close-up view of the vessel and cylindricalstructure of FIG. 2.

FIG. 4 illustrates an example of tension values over time.

FIG. 5 illustrates an example of tension values over time.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 2, in one embodiment of the invention, System 200is illustrated. X axis 202, Y axis 204, and Z axis 206 are all defined.System 200 includes vessel 210 connected to cylindrical structure 214,and cylindrical structure 214 is connected to bottom 216 of water 212.Currents 218 a, 218 b, and 218 c encounter cylindrical structure 214,causing vortexes 220 a, 220 b, and 220 c, and VIV 222 a, 222 b, and 222c. Vessel 210 includes tension monitor 230, tensioner 232, andcontroller 240. Sensors 234 a, 234 b, and 234 c are provided oncylindrical structure 214, which measure VIV and/or current.

Vortex induced vibration (VIV) is defined herein is a vibration having agiven displacement and frequency of a structure caused by the vortexeswhich are caused by an ambient current. The VIV “level” is a function ofthe displacement and the frequency of the vibrations, with higherdisplacements and higher frequencies causing higher tensions, stresses,and/or strains, and lower displacements and lower frequencies causinglower tensions, stresses, and/or strains. It is generally desirable tolower the displacement and/or the frequency of VIV in a structure, forexample to extend the structure's fatigue life.

In some embodiments, the level of VIV is calculated by averaging theacceleration of the structure over the length of the structure. Forexample, for a structure having a single accelerometer providing anoutput of 2 meters per second squared (m/s²), the VIV value would be 2m/s². In another example, for a 50 m structure having fiveaccelerometers (every 10 m) providing outputs of 1, 2, 3, 4, and 5 m/s²,the VIV value would be the average of 3 m/s². In some embodiments, theacceleration can be calculated from an accelerometer. In someembodiments, the acceleration can be calculated from one or more of thebending stress, velocity, displacement, wind or current, and/or dynamictension.

In some embodiments, the level of VIV is calculated at a given locationof the structure, for example at location with high stress concentrationfactors and/or substandard welds. For example, for a 50 m structurehaving five accelerometers (every 10 m) providing outputs of 1, 2, 3, 4,and 5 m/s², the given location with a high stress concentration factorregisters the value of 4, so the VIV value would be 4 m/s². Thislocation with the stress concentration factor would be the location toreduce the VIV level.

In some embodiments, the level of VIV is calculated at a given area ofthe structure, for example at an area that has had more fatigue damagethan the rest of the structure, in order to balance the fatigue damagealong the length of the structure and improve the overall life of thestructure. For example, for a 50 m structure having five accelerometers(every 10 m) providing outputs of 1, 2, 3, 4, and 5 m/s², the given areathat has had more fatigue damage registers the value of 2, so the VIVvalue would be 2 m/s². This area that has had more fatigue damage wouldbe the area to reduce the VIV level.

Referring now to FIG. 3, a more detailed view of vessel 210 andcylindrical structure 214 is provided. Tension monitor 230 is connectedto cylindrical structure, and is adapted to monitor the level of tensionon cylindrical structure 214 over time.

Tensioner 232 is also connected to cylindrical structure 214, and isadapted to selectively increase or decrease the tension on cylindricalstructure 214. Sensors 234 a, 234 b, and 234 c are provided oncylindrical structure 214, and are adapted to provide a measurement ofthe movements of cylindrical structure 214 (for example VIV) and/or ameasurement of currents 218 a, 218 b, and 218 c. Sensor 234 d is adaptedto provide a measure of movement of vessel 210, and/or the ambientcurrent. Controller 240 is adapted to receive input from tension monitor230, sensors 234 a-234 d, and to provide output to tensioner 232, toselectively increase and/or decrease the tension on cylindricalstructure 214, as necessary, to control VIV.

In operation, the VIV is calculated, for example by using sensors 234a-234 d, and/or from tension monitor 230. In some embodiments, VIV maybe calculated by controller 240 from the movement of sensors 234 a-234 drelative to a stationary object such as bottom 216 of water 212. In someembodiments, VIV may be estimated by controller 240 from the currentmeasurements of sensors 234 a-234 d. In some embodiments, a suitablemethod of calculating VIV from the dynamic tension measurements fromtension monitor 230 and/or calculating an optimum tension value tominimize VIV is VIV calculation software commercially available fromShell Oil Company or one of its affiliates of Houston, Tex. Controller240 then outputs an optimum tension value. In some embodiments, optimumtension value may be sent to tensioner 230 which either increases ordecreases tension on cylindrical structure 214. In some embodiments, thetension may be manually adjusted on cylindrical structure 214 based onoptimum tension value from controller 240.

For example, referring to FIG. 4, an optimal tension value for thesystem 200 is 3025 newtons (N) in order to minimize the VIV oncylindrical structure 214, starting with an initial value of tension of10,000 N, one suitable algorithm would be to start off adding 1000 N oftension and determining whether the VIV is improved or worsened. In theexample illustrated in FIG. 4, since the VIV value is worsened by addingtension from 10,000 to 11,000 N, then controller 240 would adjust thetension by subtracting 1000 N of tension each time cycle untilsubtracting 1000 N creates a worse value of VIV than the previouscycle's tension value.

In this example, the tension at each cycle would be reduced from 11,000to 10,000 to 9,000 all the way down to 2,000 N, as the values wereconsistently improving from 4,000 to 3,000. The VIV value only worsenedwhen moving from 3,000 to 2,000 N. Next, the tensioner would adjustupwards at half the previous value, in this case 500 N, using the samelogic until the VIV value is worsened by adding an additional 500 N. Inthis case, the tension would be adjusted from 2,000 to 2500 to 3000 to3500, at which point the tensioner would stop adjusting upwards asmoving from 3000 to 3500 N worsens the VIV value. The process continuesby then subtracting 250 N increments, adding 125 N increments,subtracting 62.5 N increments, adding 31.25 N increments, etc., untilthe optimal tension value of 3025 N is reached, or the system restarts.

In some embodiments, using this example, the tension adjustmentscontinue until such time as the VIV value (a function of thedisplacement and frequency, discussed above) changes by at least 2 timesthe change caused by the previous tension adjustment increment, so thatcontroller 240 restarts and the initial change made is adding 1000 N,and starting the cycle over. This may indicate a change in the subseaenvironment or other conditions which would require a new optimaltension value to be iterated. For example, if changing the tension from3500 to 3250 N changes the VIV value by 2%, and then changing thetension from 3250 to 3000 changes the VIV value by 4%, then the systemwould reset, and the next change in tension would be to add 1000 Ntension to the previous value of 3000.

In another example, referring to FIG. 5, an initial tension value is3000 N, and an optimal tension value is 7750. As before, controller 240controls tensioner 232 by adding 1000 N of tension at a time, until suchtime as adding 1000 N of tension worsens rather than improves the VIVvalue. In this case, tensioner 232 with each cycle moves from 3000 to4000 to 5000, all the way to 9000 N, as moving from 8000 to 9000 N isthe first time that the VIV value worsens by adding 1000 N. Next, 500 Nincrements are subtracted, here until the tension value reaches 7000 N,as the change from 7500 to 7000 N is the first time that the VIV valuewas worsened by subtracting 500 N. Next, 250 N increments are added,then 125 N increments are subtracted, then 62.5 N increments added, etc,until the optimal value of 7750 N of tension is reached.

In some embodiments of the invention, the system will reset at such timeas the VIV value changes by greater than 2 times the previousincremental change made by adjusting the tension value. This couldindicate a change in subsea conditions, such as a change in thecurrents.

In some embodiments of the invention, the cycle time between incrementsis set at about 0.5 to 5 minutes, for example at about 1 minute, toallow sufficient time to take VIV measurements, and to allow the changein tension to take effect on cylindrical structure 214.

In some embodiments of the invention, cylindrical structure 214 maychange its response modes of vibration due to very small changes incurrents 218 a-218 c. In some embodiments, small changes in tension cancause changes in the response mode of vibration of cylindrical structure214. These changes in mode may be accompanied by a period of lowdisplacement while cylindrical structure 214 transitions from one modeto another, akin to the vibration stopping and then restarting in adifferent response mode.

In some embodiments of the invention, active tension control may be usedto control/reduce VIV for significant durations to substantially improvethe fatigue life of cylindrical structure 214 immersed in currents 218a-218 c.

In some embodiments of the invention, cylindrical structure 214 has anatural frequency of f_(n), where n is the mode number (i.e. f₁ is thenatural frequency of the first bending mode in a given direction). Thenatural frequency is controlled by an equation that consists of atension term as well as a material stiffness term. For a long structure(such as deepwater risers, cables, umbilicals, tendons, etc.), thetension term is usually significantly larger than the material stiffnessterm, so that changes in the tension significantly affect the naturalfrequency. In this case, if the change in tension is sufficient, it willcause a change in the response mode number. When the mode number changesthe VIV may be temporarily reduced.

In some embodiments of the invention, VIV 222 a-222 c can be measured bya) measurement of structural motions; b) measurement of dynamic tension;c) measurement of an ocean current thought to produce VIV; or d) acombination of a) through c). Using a), both the frequency anddisplacement (at least at the measurement points) are known. If only b)is used, then the frequency may be known and the displacements may beinferred from the dynamic tension range. An analytical or computationalmodel of the riser can be used to relate the dynamic tension to theriser displacement, for example, VIV calculation software commerciallyavailable from Shell Oil Company or its affiliates.

In some embodiments of the invention, a method for active control of VIVthru tension control includes: (1) Input of the structural motionmeasurement and/or dynamic tension measurement, for example sampled at afrequency sufficient to approximate the vibration. (2) Conversion of thestructural motion measurements or dynamic tension measurements toestimates of vibration amplitude and/or frequency (frequency is notnecessary), if the frequency is known, a structural dynamics model ofthe riser is used to estimate the mode number (optional). Note that acurrent measurement can also be used to estimate the mode provided anaccurate VIV model is used. (3) The required tension is then computed.(4) The tension is then adjusted. Steps 1-4 are repeated as often asdeemed necessary or desired.

In some embodiments of the invention, an active control VIV mitigationsystem 200 includes: (1) a floating or fixed structure 210 for producinghydrocarbons (the offshore platform); (2) one or more longstructures/tubulars 214 in tension; (3) a tensioner system 232 forcontrolling/adjusting the tension of the tubular; (4) a measurement ofthe tension 230 that is fed electronically into a computer; (5) acomputer 240 that determines the required amount of tension adjustmentto mitigate the vortex-induced vibration motion of the tubular(s) usinga preset automatic algorithm; and (6) a mechanism 240 to feed therequired tension adjustment back to the tensioner system 232.

In some embodiments of the invention, structure 214 may have differentnatural frequencies for different directions of vibration. In someembodiments, vessel 210 will have more than one tubular. In someembodiments, a single computer 240 can compute the required amount oftension adjustment needed for VIV mitigation for multiple tubulars. Insome embodiments, a measurement of currents 218 a-218 c may also be fedinto computer 240 to improve system accuracy. In some embodiments, localmeasurements of tubular strain may also be fed into the computer 240 toimprove system accuracy. In some embodiments, tension adjustments aredone automatically. In some embodiments, system 200 may have safetyprecautions in the form of mechanical or electrical hardware thatrestricts the magnitude and/or rate of the tension adjustments to safelevels.

In some embodiments of the invention, vessel 210 may be a floating oilplatform, for example a fixed platform, a tension leg platform, a spar,or a drilling rig.

In some embodiments of the invention, structure 214 may be a mooringline, riser, a tubular, or any other structure subject to current orwind. In some embodiments, structure 214 may have a diameter of about0.1 to about 5 meters, and a length of about 10 to about 10,000 meters(m). In some embodiments, structure 214 may have a length to diameterratio of about 100 to about 100,000. In some embodiments, structure 214may be composed of about 50 to about 300 threaded tubular sections, eachwith a diameter of about 10 cm to about 60 cm and a length of about 5 mto about 50 m, and a wall thickness of about 0.5 cm to about 5 cm.

In some embodiments of the invention, tension monitor 230 may be acommercially available load cell.

In some embodiments of the invention, tensioner 232 may be acommercially available ram style tensioner.

In some embodiments of the invention, controller 240 may be acommercially available topside computer.

In some embodiments, the VIV level may be minimized by periodicallychanging the tension by at least about 5%, for example about 10%. Forexample, a riser system having an acceptable tension range of 80 to 125kN may start with a tension of 100 kN. In the first time period, thetension can be increased to 115 kN, then in the second time period,decreased to 90 kN, then increased to 110 kN, and then subsequentlydecreased and increased by at least about 10% in each time period tominimize VIV, for example by changing the mode of the riser. Thecontroller 240 may be programmed to stay within the acceptable range,increase or decrease by a minimum percentage, and make an increase ordecrease each time the VIV level increases over a given threshold.

In some embodiments, vessel 210 may have multiple structures 214attached, for example about 5 to 30, or about 10 to 20. For example, ifsystem 200 has twenty structures attached, vessel 210 has a maximumtension which can be applied to all twenty structures while stillmaintaining a safe environment. If the maximum tension which can beapplied to vessel 210 is 10,000 kN, then the average maximum tension perstructure is 500 kN. Controller 240 may be programmed to keep totaltension on vessel 210 under 10,000 kN, while minimizing the VIV level onall 20 structures.

In some embodiments of the invention, there is disclosed a systemcomprising a structure in a body of water, a vortex induced vibrationmonitoring system, adapted to monitor a vortex induced vibration levelof the structure, a tensioner connected to the structure, and acontroller adapted to control the tensioner to adjust the tension on thestructure to optimize the vortex induced vibration value of thestructure. In some embodiments, there is a vessel connected to thestructure, where the vessel is floating in the body of water. In someembodiments, the structure is selected from the group consisting ofrisers and mooring lines. In some embodiments, the vortex inducedvibration monitoring system includes a plurality of sensors on thestructure. In some embodiments, the vessel includes an oil platform. Insome embodiments, the structure includes one or more strakes and/orfairings adapted to lower the vortex induced vibration value of thestructure.

In some embodiments of the invention, there is disclosed a method ofcontrolling vortex induced vibration of a structure in a body of waterincluding monitoring a level of vortex induced vibration in thestructure, and adjusting the tension in the structure to minimize thelevel of vortex induced vibration. In some embodiments, the method is aniterative process that continues for a plurality of time cycles. In someembodiments, the structure is selected from risers and mooring lines. Insome embodiments, monitoring the vortex induced vibration includesmeasuring a value from a plurality of sensors on the structure. In someembodiments, the structure includes one or more strakes and/or fairingsadapted to lower the vortex induced vibration value of the structure. Insome embodiments, the method is an iterative process that continues fortime cycles of about 0.5 to about 5 minutes. In some embodiments, themethod also includes calculating an optimal tension value for thestructure. In some embodiments, calculating an optimal tension value forthe structure is an iterative process that continues for time cycles ofabout 0.5 to about 5 minutes, for example about 1 minute.

In some embodiments of the invention, there is disclosed an apparatusfor minimizing vortex induced vibration of a structure, comprising ameans for calculating a level of vortex induced vibration in thestructure, and a means for calculating a tension in the structure tominimize the level of vortex induced vibration.

In some embodiments of the invention, there is disclosed a system forcontrolling vortex induced vibration, including a cylindrical structurewithin a body of water, a means for monitoring the level of vortexinduced vibration of the cylindrical structure, a means for optimizingthe level of vortex induced vibration of the cylindrical structure. Insome embodiments, the cylindrical structure is connected to a vessel isfloating in the body of water. In some embodiments, the cylindricalstructure is selected from the group consisting of risers and mooringlines. In some embodiments, the means for monitoring the level of vortexinduced vibration includes a plurality of sensors on the cylindricalstructure. In some embodiments, the means for monitoring the level ofvortex induced vibration includes a system for calculating the level ofvortex induced vibration from a level of tension of the cylindricalstructure. In some embodiments, the cylindrical structure includes oneor more strakes or fairings, for example about 10 to about 100, adaptedto lower the vortex induced vibration value of the structure. Suitablestrakes are disclosed in U.S. Pat. No. 6,561,734, which is hereinincorporated by reference in its entirety. Suitable fairings aredisclosed in U.S. Pat. No. 6,223,672, which is herein incorporated byreference in its entirety.

In some embodiments of the invention, there is disclosed a system foroptimizing vortex induced vibration of a cylindrical structure in a bodyof water, including a system for measuring and calculating vortexinduced vibration values of the cylindrical structure, a tensioneradapted to change the tension on the cylindrical structure, and acontroller adapted to change the tension on the cylindrical structure inorder to optimize the vortex induced vibration value. In someembodiments, the cylindrical structure is connected to a vessel isfloating in the body of water. In some embodiments, the cylindricalstructure is selected from the group consisting of risers and mooringlines. In some embodiments, the system for measuring and calculatingvortex induced vibration values includes a plurality of sensors on thecylindrical structure. In some embodiments, the system for measuring andcalculating vortex induced vibration values includes calculating thelevel of vortex induced vibration from a level of tension of thecylindrical structure. In some embodiments, the cylindrical structureincludes one or more strakes or fairings adapted to lower the vortexinduced vibration value of the cylindrical structure.

Those of skill in the art will appreciate that many modifications andvariations are possible in terms of the disclosed embodiments,configurations, materials and methods without departing from theirspirit and scope. Accordingly, the scope of the claims appendedhereafter and their functional equivalents should not be limited byparticular embodiments described and illustrated herein, as these aremerely exemplary in nature.

1. A system comprising: a structure; a vortex induced vibrationmonitoring system, adapted to monitor a vortex induced vibration levelof the structure; a tensioner connected to the structure; and acontroller adapted to calculate an optimal tension value on thestructure to minimize the vortex induced vibration value of thestructure, and adapted to adjust the tensioner to the optimal tensionvalue.
 2. The system of claim 1, further comprising a vessel connectedto the structure, wherein the vessel is floating in a body of water. 3.The system of claim 1, wherein the structure is selected from the groupconsisting of risers and mooring lines.
 4. The system of claim 1,wherein the vortex induced vibration monitoring system comprises aplurality of sensors on the structure.
 5. The system of claim 1, furthercomprising a vessel connected to the structure, wherein the vesselcomprises an oil platform.
 6. The system of claim 1, wherein thestructure comprises one or more strakes and/or fairings adapted to lowerthe vortex induced vibration value of the structure.
 7. The system ofclaim 1, wherein the tensioner is adapted to be manually adjusted. 8.The system of claim 1, wherein the tensioner is adapted to beautomatically adjusted based on the tension value calculated by thecontroller.
 9. A method of controlling vortex induced vibration of astructure comprising: monitoring a level of vortex induced vibration inthe structure; adjusting the tension in the structure to minimize thelevel of vortex induced vibration; and calculating an optimal tensionvalue for the structure.
 10. The method of claim 9, wherein the methodis an iterative process that continues for a plurality of time cycles.11. The method of claim 9, wherein the structure is selected from thegroup consisting of risers and mooring lines.
 12. The method of claim 9,wherein monitoring the vortex induced vibration comprises measuring avalue from a plurality of sensors on the structure.
 13. The method ofclaim 9, wherein the structure comprises one or more strakes and/orfairings adapted to lower the vortex induced vibration value of thestructure.
 14. The method of claim 9, wherein the method is an iterativeprocess that continues for time cycles of 0.5 to 5 minutes, for example1 minute.
 15. The method of claim 9, wherein calculating an optimaltension value for the structure is an iterative process that continuesfor time cycles of 0.5 to 5 minutes, for example 1 minute.
 16. Anapparatus for minimizing vortex induced vibration in a structure,comprising: a means for calculating the level of vortex inducedvibration of the structure; a means for calculating an optimal level oftension in the structure to minimize the vortex induced vibration; and atensioner means adapted to apply the optimal level of tension.
 17. Theapparatus of claim 16, wherein the means for calculating the level ofvortex induced vibration comprises a plurality of sensors on thestructure.
 18. The apparatus of claim 16, wherein the means forcalculating the level of vortex induced vibration comprises calculatingthe level of vortex induced vibration from a level of tension of thestructure.
 19. The apparatus of claim 16, wherein the means forcalculating the level of vortex induced vibration comprises calculatingthe level of vortex induced vibration from a level of current or windabout the structure.
 20. The apparatus of claim 16, wherein the meansfor calculating the level of vortex induced vibration and the means forcalculating an optimal level of tension are adapted to iterativelycalculate, for example every 0.5 to 5 minutes.